The present invention relates to a wireless communication apparatus, a mobile terminal, and a wireless communication method, and in particular relates to a wireless communication apparatus, mobile terminal, and wireless communication method, in which a plurality of antennas are provided and in which switching is performed between first multi-antenna transmission control that performs multi-stream transmission (for example, MIMO transmission control) and second multi-antenna transmission control that performs single-stream transmission (for example, AAA transmission control).
In multi-antenna technology using multiple antennas, there are both MIMO (multiple-input, multiple-output) transmission control to transmit multiple streams, and AAA (adaptive array antenna) transmission control to transmit a single stream.                MIMO        
FIG. 18 shows the configuration of a MIMO multiplexed transmission system, where TRX is a transmitting station and REC is a receiving station. The same number of data streams D0 to DM-1 as the number of transmission antennas M pass through data modulation unit, D/A conversion unit, quadrature modulation unit, frequency up-conversion unit, and similar in the respective transmission apparatus (TRX0 to TRXM-1) 10 to 1M-1, and are transmitted from each of the antennas 20 to 2M-1. Signals transmitted from the antennas 20 to 2M-1, disposed such that there is no mutual correlation, pass through independent fading channels hnm (m=0 to M−1, n=0 to N−1), and after space multiplexing are received by N receiving antennas 30 to 3N-1. Signals received by each receiving antenna pass through frequency down-conversion unit, quadrature detection unit, A/D conversion unit and similar in the reception apparatus (REC0 to RECN-1) 40 to 4N-1, to generate received data streams y0 to yN-1. Each of the received data streams is the result of multiplexing M transmitted data streams, and so in the data processing portion 5 signal processing of all the received data streams is performed, so that the transmitted data streams D0 to DM-1 can be separated and reproduced.
Signal processing algorithms to separate the transmitted data streams D0 to DM-1 from the received signals include such linear algorithms as ZF (Zero-Forcing) and MMSE, employing the inverse matrix of a channel correlation matrix, and nonlinear algorithms, of which BLAST (Bell Laboratories Layered Space-Time) is representative. In addition, MLD (Maximum Likelihood Decoding) and other methods which do not employ operations on the inverse matrix of a correlation matrix are also known. The MLD algorithm is here explained. If the transmitted data streams are represented by an M-dimensional complex matrix, and the received data streams by an N-dimensional complex matrix, then the following equations obtain.
                              Y          =                      H            ·            D                          ⁢                                  ⁢                  H          =                      [                                                                                                                              h                        00                                            ·                                              h                                                  01                          ⁢                                                                                                                                                                      ⁢                    …                    ⁢                                                                                  ⁢                                          h                                                                        0                          ⁢                          M                                                -                        1                                                                                                                                                                                    h                                              10                        ⁢                                                                                                                                        ⁢                                                                                  ⁢                    …                    ⁢                                                                                  ⁢                                          h                                                                        1                          ⁢                          M                                                -                        1                                                                                                                                          …                                                                                                                        h                                                                        N                          -                          10                                                ⁢                                                                                                                                        ⁢                    …                    ⁢                                                                                  ⁢                                          h                                              N                        -                                                  1                          ⁢                          M                                                -                        1                                                                                                                  ]                          ⁢                                  ⁢                  D          =                                    [                                                                    D                    0                                    ·                                      D                    1                                                  ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  D                                      M                    -                    1                                                              ]                        T                          ⁢                                  ⁢        Y        =                              [                                                            y                  0                                ·                                  y                  1                                            ⁢                                                          ⁢              …              ⁢                                                          ⁢                              y                                  N                  -                  1                                                      ]                    T                                    (        1        )            
The MLD algorithm is a method which does not employ operations on the inverse matrix of the correlation matrix; the transmitted data streams (transmission vector) D are inferred from the following equation.{circumflex over (D)}=argmin∥Y−H·D∥2  (2)
Here, if the number of signal points of modulated data input to each of the M antennas is Q, then there exist QM combinations of transmission vectors D. In QPSK, Q=4, in 16QAM, Q=16, and in 64QAM, Q=64. The MLD algorithm is a method in which QM transmission vector candidates (replicas) are generated, computations using the above equations are performed, and the replica for which the result is smallest is inferred to be the input data.                AAA        
An array antenna comprises a plurality of antenna elements, arranged in different spatial positions in a straight line. Technology in which the signals input to each of the antenna elements in this array antenna are weighted, to direct a transmission beam toward a mobile terminal, is called AAA (Adaptive Array Antenna) control.
FIG. 19 shows the configuration of a wireless apparatus which uses AAA control for data transmission and reception. The array antenna 30 receives signals, which are input to a baseband and digital signal processing portion 31. The signal processing portion 31 performs signal processing for each antenna element, and outputs complex digital reception data. The arrival direction estimation portion 32 estimates the arrival direction of signals, using the complex digital reception data for each antenna element. The beamformer (reception beamformer) 33 uses the estimated values for the arrival direction of signals, acquired from the arrival direction estimation portion 32, to form a beam having a peak in the signal source direction. That is, the beamformer 33 extracts the desired signals (signals from the mobile terminal), while suppressing interference, noise and similar, and sends the signals to the channel reception portion 34. The channel reception portion 34 uses well-known methods to perform reception processing, and demodulates and outputs the received data.
On the other hand, when transmitting data to the mobile terminal, the transmission beamformer 35, upon input of transmission data from the transmission portion 36, forms a transmission beam pattern such that the peak is directed in the direction estimated by the arrival direction estimation portion 32, and inputs complex digital transmission signals to the baseband and digital signal processing portion 37. The signal processing portion 37 converts the complex digital transmission data into wireless signals, which are input to each of the antenna elements of the array antenna 38. As a result, a beam is radiated toward the receiving station, and the error rate can be reduced. The array antennas 30 and 38 can be made a common antenna.
However, MIMO, AAA, and other multi-antenna technologies have not been introduced into portable telephone systems of current services, and their future application is being studied. There have however been reports of the use of AAA in PHS, and of MIMO in wireless LANs.
These multi-antenna technologies have been studied, and various methods have been proposed, targeting introduction into future mobile communication systems. In MIMO transmission, when as described above there is almost no correlation between signals from the transmission antennas to the reception antennas, high-speed data transmission can be achieved, and AAA transmission is effective when the correlation between signals from the transmitting antennas to the receiving antennas is close to 1. Drawing on these facts, as a first technology of the prior art, a method has been proposed of switching between MIMO transmission when the correlation between antennas is low, and AAA transmission when the correlation is high (see JP2004-194262A). In the first technology of the prior art, spatial correlation values between antennas and the received signal SIR (Signal to Interference Ratio) are measured in the receiving apparatus, and using these spatial correlation values and SIR values, the communication capacity for each transmission method (MIMO, AAA, or similar) is determined, the transmission apparatus is notified of the communication method for which the communication capacity is largest, and the transmission apparatus performs transmission according to this transmission method. However, the first technology of the prior art has the problems of requiring calculation of the spatial correlation values between antennas and SIR values, and of having a complex configuration. Further, switching to the optimum transmission method is not possible until the transmission method with the largest communication capacity has been determined, so that there is the problem of delayed switching.
As the second technology of the prior art, a method has been proposed in which the SIR is measured, and when the SIR is equal to or less than a threshold, switching is performed from MIMO to STC (Space-Time Coding) (see JP2004-072624A). STC is a transmission technology in which encoded substreams are transmitted simultaneously from a plurality of antennas, and combines encoding and transmission diversity. That is, in STC the transmission apparatus performs spatial-time encoding processing of the data stream, to generate a plurality of mutually different data streams, and performs wireless transmission of these data streams simultaneously, at the same frequency, from a plurality of transmission antennas. The reception apparatus uses pilot signals included in the reception signals to perform propagation path estimation corresponding to each pair of transmission and reception antennas, and based on the estimated propagation path characteristics, decodes the reception signals, in which information bits and parity bits are superposed.
In the second technology of the prior art, the reception SIR values of the plurality of reception antennas are calculated in the reception apparatus, the SIR values of all the reception antennas are evaluated and the optimum transmission method determined, and the transmission apparatus is instructed to perform transmission using this transmission method; although the configuration is not as complex as in the first technology of the prior art, there is the problem of delayed switching.
When introducing multi-antenna technology into a mobile communication system, MIMO communication targets improvement of the transmission rate (higher throughput), while AAA transmission targets improvement of communication quality (SNR, Signal-to-Noise Ratio). When the overall transmission power is held constant, in MIMO transmission power is allocated to numerous streams to realize a high transmission rate, whereas in AAA, by forming a single stream into a highly directional beam, radio waves can be transmitted farther, so that long-distance communication is possible. In other words, in MIMO transmission high-speed transmission can be achieved, but the communication distance cannot be lengthened. And in AAA transmission, the communication distance can be lengthened, but the transmission rate is limited.